Semi-conductive electrode system

ABSTRACT

894,672. Semi-conductor devices. PHILIPS ELECTRICAL INDUSTRIES Ltd. June 27, 1958, No. 20704/58. Drawings to Specification. Class 37. A wire, at least the envelope of which is aluminium, and which is coated with a layer of oxide of sufficient thickness to enable fusion of the aluminium to take place without breakage of the oxide layer, is fused to a semi-conductor material. The oxide layer should have a thickness of from 3 to 40 microns and the semiconductor material and the material inside the aluminium envelope of the wire may be germanium or silicon, and tin may be used to connect the semi-conductor device to a base-plate. The device may be a diode or a transistor and in one embodiment (Fig. 3, not shown) it may be sealed inside an evacuated casing. A device according to the invention may be constructed by degreasing an aluminium wire by washing in caustic soda solution, nitric acid and then water and then oxidizing the surface either by dipping in a solution of H3PO3, CrO3, NH4HF2, and (NH4)2HPO3 in water or by subjecting the wire to anodic treatment at 60 volts D.C. in a solution of oxalic acid in water. The end of the wire may then be cut and polished and held in contact with a silicon body by means of a graphite jig (Fig. 2, not shown), while the whole device is heated in an oven to cause fusion to occur. Copper wire or cable may then be spotwelded to the aluminium wire at a point some distance away from the semi-conductive body.

Jan. ,2, 1962 Filed June 27, 1958 Fig.1

INVENTOR LUDOVICUS AUGUSTINUS LAMBERTUS ESSELING BY A,

AGEN

United States Patent are Filed June 27, 1958, Ser. No; 745,032 Claimspriority, application Netherlands July 1,1957 9 Claims. (Cl. 317-234)This invention relates to semi-conductor devices, for

example transistors or crystal diodes, having a semi-con-v ductive bodyto which oneend of at least one electrode wire is secured by fusion.

It has previously been suggested for the body, tc gether with a wireapplied thereto at one end, to be heated at a temperature such that bothelements fuse together at the contact area. i 7

When it is endeavoured in this way to secure by fusion a wire ofaluminum, it appears, more particularly if the semi-conductive bodyconsists of silicon, that heating above the melting point of aluminum isnecessary, so that the wire, although it is covered with a natural oxidelayer, flows together into a drop or at least loses its filamentaryform. Electrodes of aluminum are therefore manufactured by applying byfusion either pellets the electrodematerial, in the present case in thewire,

or thin discs of aluminum. However, it is ditficult for the resultantelectrodes to be provided with supply wires,

since, as is well-known, soldering to aluminum is in practice impossibleas a result of the aforementioned natural oxide layer, whichspontaneously forms on aluminum clue to action of the oxygen in the air.

An object of the invention is inter alia to provide a construction inwhich these disadvantages do not occur,v

and also amethod by which this construction may be realized in a simplemanner.

may restrict penetration of the electrode material into theSemi-conductive body in a manner known per se.-

Such a wire may be manufactured by filling an aluminum tube with theother material and subsequently reducing the cross-section of the wholeby swaging or expanding. In order that the invention may be'readily'carried into effect, several embodiments will now be explainedin detail, by way of example, with reference to the accompanyingdrawing, in which:

FIG. 1 shows diagrammatically a side-view of a diode; FIG. 2 shows adevice for applying wires to semioonductive bodies by fusion. 7

FIG. 3 is a cross-sectional view of a semi-conductor device according tothe invention. The diode shown in FIG. 1 comprises a thin silicon disc 1of conductivity, to which an aluminum wire 2 is secured by fusion in anoven at a temperature preferably higher than the melting point ofaluminum, for instance 8009 C. The oven temperature may however inprinciple be lower than the melting point of aluminum, but higher thanthe eutectic temperature of the silicon-aluminum mixture. The heatingtakes place in reducing surroundings, for example in a mixture ofnitrogen and hydrogen. A small addition of hydrochloricacid gas as aflux enhances adhesion of the wire to the body. The wire is providedwith a genetic oxide layer 3 which ensures that the form of the wiresubstantially does not change during heating and that flowing-out of thealuminum is restricted within certain limits. The strengthened oxidelayer is naturally not present at the area wherethe aluminum must fusetogetherwith the silicon. [The reoxide sheath. The thickness of thewire'is not essential According to the invention, the wireof which atleast t the envelope consists of aluminum, has so thick a genetic oxidelayer on its cylindrical outer surface that fusion of the aluminum ispossible without breakage of the oxide layer.

Said oxide layer preferably has a thickness comprised between 5 and 40microns. Consequently, this layer is much thicker than the natural oxidelayer already present on aluminm and which has a thickness'of only 0.1to 0.2 gt. 1

An oxide layer of sufficient thickness maybe obtained in a simple mannerby chemical means or by anodic'.

oxidation. a

The method according to the invention is characterized in that a wire,of which at least the envelope consists of aluminum, is provided with agenetic oxide, layer of a thickness such that fusion of the aluminum ispossible without breakage of the oxide. layer, whereupon one end of thiswire is secured by fusion to a semi-conductive body. It will be evidentthat it is also possible for short pieces to be cut from a long oxidizedwire.

It is to be noted that a genetic oxidelayer is to be understood to meana layer, the aluminum content of which substantially originates from theinitial wire. The term oxide layer is to be understood to mean alsolayers consisting of oxide hydrates, such as the compound Al O -H O, andfurthermore oxide layers strengthened by impregnation or in another way,as will be discussed more fully hereinafter.

As a rule, the'wire homogeneously consists of aluminum. However, it ispossible to utilize a wire having an envelope of aluminum and a core ofa different material. During fusion, the envelope and the core may fusetogether. I

The core may consist, for example, of semi-conductive material, such asgermanium or silicon. Its presence in to the invention and is usuallyfrom lOOmicrons to several millimetres in practice. a Y

I The, semi-conductive body is means of tin solder 4. v

The use of a'flux, such as hydrochloric-acid gas, is greatly simplifieddue to the presence of the oxide layer, since such fluxes usually alsochemically attack the aluminum to aconsiderable extent, which phenomenonis greatly suppressed by the presence of the strengthened oxide layer. p

The aluminum wire with its strengthened. oxide layer may be manufacturedin various ways. known per se, that is to say by purely chemical meansor an anodic oxidation. The last-mentioned method usually gives thickeroxide layers than the first-mentioned and is therefore preferable forcomparatively thick wires.

One example of each method will now be given hereinafter.

secured to a base 5 by I v I Pure aluminum wire is de'greased for half aminute in a bath-containing 15 gms. of sodium hydroxide to gms.-of waterat a temperature of 20 C. After rinsing in water, any residues of thisbath are neutralized in a solution of nitric acid having a specificgravity of 1.25 at room temperature for 1 minute, whereafter the wire isrinsed in water. Such removal of grease is not necessary for wire whichis very clean.

Subsequently, the wire is dipped for 4 minutes in an oxidizing bath ofthe conventional type containing for example,

55 cos. of phosphoric acid H PO density 1.7

22 gms. of chromic acid CrO 3.3 gms. of acid ammonium fluoride NH HF 2.2gms. of acid diammonium phosphate (NH4)2HPO3 1000 gms. of water at atemperature of 50 C. Of course also other oxidizing baths can be used.

3 The wire is then carefully rinsed and dried. The thickness of theoxide layer is from 3 to 5 microns.

Aluminum wire degreased as described in Example I is subjected at 20 C.to an anodic treatment at 60 volts (direct voltage) for one hour. Thebath was a conventional one, containing 50 guts. of oxalic acid C H H 2HO per litre. Also other oxidizing baths are suitable. The thickness ofthe resultant oxide layer was about 40 microns.

Several such purely chemical or electro-chemical methods are known andit is not essential to the invention which method is used, provided theresultant layer has suflicient thickness and, of course, does notcontain constituents detrimentally'atiecting the performance of thesemi-conductor device.

From the oxidized wire obtained in accordance with Example i or II,there are cut oif pieces each having a length of mms, which in a jig ofgraphite are placed on the bodies of silicon (see KG. 2). The jig isconstituted by two parts 11 and 12, which fit into each other. The part12 has recesses 13, by which the semi-conductive bodies are kept inposition, and perforations 14, by which the pieces of Wire 10 arecentred with respect to the bodies.

After the above-described heating in an oven, the pieces of wire arefound to have retained their initial form. Subsequently, thesemi-conductive bodies are after-etched in the usual manner, for examplewith the use of hydrofluoric .acid, the strengthened oxide-layer thenagain having the advantage of protecting the aluminum itself againstchemical attack.

It is to be noted that the pieces of Wire 10, before being applied byfusion, are preferably provided with a fiat end by polishing or filing.

The pieces of wire 10 may be connected by" spot- Welding to a piece ofcop-per Wire or copper cable which may be connected by soldering to aconnecting member.

During spot-welding, the strengthened oxide layer is naturally brokenthrough, but this is not dangerous, the operation being effected at somedistance from the semiconductive body.

FIG. 3 shows an example of such a'structure applied to a diode in anevacuated envelope. The envelope comprises a base with a threaded stud31 for attachment, and a cap 32. The cap 32 contains a glass leadthroughinsulator 33 containing a small metallic tube 34. Whereas the aluminumwire 10 is secured by spot-Welding to a copper Wire '36 at 35. Afterthis wire has been passed through the tube 34, the base 30 and the cap32 are secured together by means of flanges 37 and 38, for example byWelding. Subsequently, the envelope is fully closed due to the copperwire 36 being soldered in the tube 34.

As previously mentioned, it is alternatively possible to utilize a wirehaving an envelope of aluminum and a core of different material.The-latter may contain or consist of an active impurity such as a donorand/or an acceptor, but also semi-conductive material and more Thesilicon body 1 is soldered to the base 30,

particularly the same material as that of the body to which the wire issecured.

What is claimed is:

1. An electrode comprising a wire-like ele ent adapted for fusion at oneend to a semiconductive body, the outer Wire portions consistingessentially of aluminum and containing a genetic oxide layer'having athickness between 3 and 40 microns enabling fusion of the alu minumwithout destruction of the layer.

2. A semiconductor device comprising a semiconductive body, and aWire-like element fused at one end to the semiconductive body, the outerwire portions consisting essentially of aluminum and containing agenetic aluminum oxide layer having a thickness between about 3 and 40microns. A 3. A device as set forth in claim 2 wherein the wirelikeelement comprises a core of a semiconductive material, an envelope ofaluminum, and a surrounding aluminum oxide coating.

4. A device as set forth in claim 2 wherein the wirelike elementcomprises a core of an impurity material, an envelope of aluminum, and asurrounding aluminum oxide coating.

5. A semiconductor device comprising a semiconductive silicon body, andan aluminum wire-like element fused at one end to the semiconductivebody, the outer wire portions containing a genetic aluminum oxide layerhaving a thickness between about 5 and 40 microns.

6. A method of making a semiconductor device comprising, providing awire whose outer portions consist essentially of aluminum, subjectingsaid Wire to an oxidizing treatment'to provide on the aluminum portionsa genetic'oxide' layer having a thickness enabling the aluminum to befused without desruction of the oxide layer, and using an end of thesaid Wire to a semiconductive body. 7. A method of making asemiconductor device comprising providing a wire whose outer portionsconsist essentially of aluminum, subjecting said Wire to an oxidizingtreatment to provide a'genetic oxide layer having a thickness of atleast about 5 microns, placing a cut end of the said wire in contactwith a semiconductive body, and heating the assembly at a temperature atwhich I the wireend fuses to the semiconductive body.

8. A method of making a semiconductor device comprising providing analuminum wire, subjecting said Wire to an oxidiring'treatment to providethereon a genetic aluminum oxide layer having a thickness between 3 and40 microns, placing a cut end of the wire in contact with a siliconsemiconductive body, and heating the assembly in an oven at atemperature above the eutectic temperature of the silicon-aluminum alloyand at which the 2,840,770 Jackson June 24, 1958 2,844,770 Vessem July22, 1958 2,906,932 Fedotowsky et a1. Sept. 29, 1959 UNITED STATES PATENTOFFICE e CERTIFICATE OF CORRECTION January 2 1962 Patent Nm-O OESJlLudcvicus Augustinus Lambertus Eseeling s in the above numbered pat- Itis hereby certified that error appear tters Patent should read as entrequiring correction and that the said Le corrected below.

Column 2,, line 17 after of insert n-type line 65,, for H PQ read B 190line 68 for NH )2HP0 read QNH D HPO column 3,, line 9 for "*C H H .2H0".

reed C204H2.2H20

Signed and sealed this 1st day of May 1962c (SEAL) Attest:

DAVID L. LADD Commissioner of Patents ERNEST W0 SWIDER Attesting Officer

2. A SEMICONDUCTOR DEVICE COMPRISING A SEMICONDUCTIVE BODY, AND AWIRE-LIKE ELEMENT FUSED AT ONE END TO THE SEMICONDUCTIVE BODY, THE OUTERWIRE PORTIONS CONSISTING ESSENTIALLY OF ALUMINUM AND CONTAINING AGENETIC ALUMINUM OXIDE LAYER HAVING A THICKNESS BETWEEN ABOUT 3 TO 40MICRONS.